Whenever I’m reaching the end of a skein, I always wonder if there’s enough yarn remaining to complete another row. Here’s a rule of thumb for that situation, from Creative Knitting:
Spread out your work, so you get the full width of the piece. With your remaining yarn, loosely extend it from one side to the other and back again. If you have enough to spread your yarn three times across the knitting, you should be fine as long as you don’t have bobbles, cables or other yarn-gobbling features. If you are binding off, you’ll need at least four lengths as this procedure takes that much more to do it properly.